diff --git a/src/client.js b/src/client.js
index 1ba3c4604..72c57d60c 100644
--- a/src/client.js
+++ b/src/client.js
@@ -946,6 +946,35 @@ MatrixClient.prototype.getGlobalBlacklistUnverifiedDevices = function() {
     return this._crypto.getGlobalBlacklistUnverifiedDevices();
 };
 
+/**
+ * Set whether sendMessage in a room with unknown and unverified devices
+ * should throw an error and not send them message. This has 'Global' for
+ * symmetry with setGlobalBlacklistUnverifiedDevices but there is currently
+ * no room-level equivalent for this setting.
+ *
+ * This API is currently UNSTABLE and may change or be removed without notice.
+ *
+ * @param {boolean} value whether error on unknown devices
+ */
+MatrixClient.prototype.setGlobalErrorOnUnknownDevices = function(value) {
+    if (this._crypto === null) {
+        throw new Error("End-to-end encryption disabled");
+    }
+    this._crypto.setGlobalErrorOnUnknownDevices(value);
+};
+
+/**
+ * @return {boolean} whether to error on unknown devices
+ *
+ * This API is currently UNSTABLE and may change or be removed without notice.
+ */
+MatrixClient.prototype.getGlobalErrorOnUnknownDevices = function() {
+    if (this._crypto === null) {
+        throw new Error("End-to-end encryption disabled");
+    }
+    return this._crypto.getGlobalErrorOnUnknownDevices();
+};
+
 /**
  * Add methods that call the corresponding method in this._crypto
  *
diff --git a/src/crypto/algorithms/megolm.js b/src/crypto/algorithms/megolm.js
index 911f2311d..42cee48ba 100644
--- a/src/crypto/algorithms/megolm.js
+++ b/src/crypto/algorithms/megolm.js
@@ -736,7 +736,9 @@ MegolmEncryption.prototype.encryptMessage = async function(room, eventType, cont
 
     // check if any of these devices are not yet known to the user.
     // if so, warn the user so they can verify or ignore.
-    self._checkForUnknownDevices(devicesInRoom);
+    if (this._crypto.getGlobalErrorOnUnknownDevices()) {
+        self._checkForUnknownDevices(devicesInRoom);
+    }
 
     const session = await self._ensureOutboundSession(devicesInRoom, blocked);
     const payloadJson = {
diff --git a/src/crypto/index.js b/src/crypto/index.js
index 879bd706d..f47d8877e 100644
--- a/src/crypto/index.js
+++ b/src/crypto/index.js
@@ -177,6 +177,7 @@ export default function Crypto(baseApis, sessionStore, userId, deviceId,
     this._deviceKeys = {};
 
     this._globalBlacklistUnverifiedDevices = false;
+    this._globalErrorOnUnknownDevices = true;
 
     this._outgoingRoomKeyRequestManager = new OutgoingRoomKeyRequestManager(
          baseApis, this._deviceId, this._cryptoStore,
@@ -1210,6 +1211,29 @@ Crypto.prototype.getGlobalBlacklistUnverifiedDevices = function() {
     return this._globalBlacklistUnverifiedDevices;
 };
 
+/**
+ * Set whether sendMessage in a room with unknown and unverified devices
+ * should throw an error and not send them message. This has 'Global' for
+ * symmertry with setGlobalBlacklistUnverifiedDevices but there is currently
+ * no room-level equivalent for this setting.
+ *
+ * This API is currently UNSTABLE and may change or be removed without notice.
+ *
+ * @param {boolean} value whether error on unknown devices
+ */
+Crypto.prototype.setGlobalErrorOnUnknownDevices = function(value) {
+    this._globalErrorOnUnknownDevices = value;
+};
+
+/**
+ * @return {boolean} whether to error on unknown devices
+ *
+ * This API is currently UNSTABLE and may change or be removed without notice.
+ */
+Crypto.prototype.getGlobalErrorOnUnknownDevices = function() {
+    return this._globalErrorOnUnknownDevices;
+};
+
 /**
  * Upload the device keys to the homeserver.
  * @return {object} A promise that will resolve when the keys are uploaded.